Lasers are used to inscribe discernable and non-discernable marks on materials, such as plastics. This technology is currently used, for example, to inscribe ophthalmic lenses in general, and Progressive Addition Lenses, in particular. A Progressive Addition Lens (PAL) is an eyeglass lens that incorporates corrections for distance vision through midrange, to near vision (usually in lower part of lens), with smooth transitions and no bifocal demarcation line. Progressive Addition Lenses are currently made of glass or plastic materials. Often the glass lenses are called “mineral lenses” while the plastic lenses are called “organic lenses”.
Progressive Addition Lenses (PAL) require inscriptions at several locations. The inscription may include reference marks for later edging of the lens, for power verification of the “add power” of the lens, for a logo, for a brand-name, for anti-counterfeiting, and for other technological reasons and applications. Typically, a lens inscription includes small marks, typically 1-2 mm in height, characters and logos that are clear, aesthetic and non-discernable.
One method of producing a plastic Progressive Addition Lens is by machining one or both of its surfaces to form an aspheric shape or shapes. When only one surface is aspherically shaped, the other surface may be spherical. This method is referred to as “Free-Form”.
The marks, although they are within the active area of the lens, must not hinder the sight of the eyeglasses wearer nor be seen by a casual observer. On the other hand, the marks must be readable, for instance, by technical and marketing personnel, by a customer who seeks the manufacturer's brand name, or by an imaging vision machine. The non-discernable mark can be observed under certain light conditions such as against the transition between light and shadow. This type of inscription is referred herein as a non-discernable inscription, also known as “semi-visible”, “invisible”, “hidden mark”, or similar names.
Laser inscription is a conventional technology for non-discernable inscription on Free-Form plastic Progressive Addition Lenses. The most commonly used lasers for this purpose (today) are the Excimer laser and the solid-state UV laser. However, these systems are very expensive.
A CO2 laser, which is a typical state-of-the art laser, is relatively inexpensive, very reliable and requires little and infrequent maintenance. CO2 lasers are used for the inscription of non-discernable marks. For instance LaserOp Ltd 6 Halapid St. Petah Tikva, 49170, Israel, manufactures LensMark™ II, which is a CO2 laser-based system for inscription of non-discernable marks, and so do others. The LensMark™ II system is suggested herein as an example, but other suitable CO2 laser based systems for inscription of materials may be used.
Compared to the laser systems which are based on Excimer and other UV lasers systems, the LensMark™ II is much more reliable and affordable in price. The LensMark™ II satisfies many requirements for the inscription of non-discernable marks. It generates acceptable non-discernable inscriptions on all plastic lenses tested so far, such as polycarbonate, CR39, Index 1.6, Index 1.67, Index 1.7, Trivex and others, as well as on selected glass lenses. The inscription is acceptably non-discernable using LensMark™ II when it inscribes the material after tinting or where tinting is not applied to the material.
However, when the tinting is performed on a plastic lens which has been previously inscribed by a CO2 laser, the marks are unacceptably visible. For example, in a typical inscription process of the LensMark™ II system alone, it is found that tinting above 12% absorption makes the inscriptions unacceptably visible.
This problem is known to those who are skilled in the art. This problem is one of the reasons that many lens producers avoid using a CO2 laser for the inscription of non-discernable marks on plastic lenses.
The inscription process of the plastic material by the CO2 laser beam, unlike the inscription process performed by Excimer or solid-state UV lasers, is a thermal or a heating-type inscription process.
The area, which is affected by the heating of the material as a result of the absorption of the laser beam, is the “Heat Affected Zone” (HAZ). This zone may include areas of material that have been removed by evaporation, areas of material that have been melted and re-solidified and areas of material that have gone through other morphological changes due to the heat.
Typically, the evaporation creates a hole, whose walls and bottom and surroundings are covered by the melted and re-solidified material. The HAZ can be described by a form of a hole with a diameter proportional to the beam's spot size, surrounded by an elevated ring. However, depending on the material and the laser beam parameters, the material might not reach the evaporation temperature anywhere within the beam cross-section and a hole is not created. The result will be a disc-like shape of elevated material. Similarly, when the beam scans the material continuously, the result may be a groove with elevated banks or a continuous line of elevated material.
The material at the HAZ may become porous, may have lower density relative to material in unaffected zones, and may occupy a larger volume. It is believed that the lower density of the material in the HAZ increases the density of the dye physically absorbed there, thus increasing the optical absorption by the dye, and consequently increasing its contrast and visibility.
Therefore, it would be desirable to provide a system and method for inscribing marks that remain non-discernable after tinting, on materials in general, and on plastic ophthalmic lenses in particular, using a CO2 laser.